1. Field of the Invention
The present invention relates to a method and apparatus for generating pulses in general and to a method and apparatus for generating alternate bipolar pulses in a local area network in particular.
2. Description of Prior Art
A local area network, as its name implies, is a network of transmitting and receiving stations which are coupled together by means of a communications link within a relatively small area.
The transmitting and receiving stations may be combined, as they often are, in a single unit. When they are so combined, they are often called transceivers. The communications link which is used to couple the transceivers is usually a conventional coaxial transmission line. The transmission line typically used comprises a relatively low characteristic impedance, e.g. 50-100 ohms, and may be several kilometers long.
In a typical local area network, the transceivers are coupled in parallel to the transmission line at spaced points along the line. In networks in which the transceivers are used for coupling digital terminals, the transceivers comprise pulse transmitters and receivers for transmitting and receiving pulse streams corresponding to the data being transmitted between them.
Heretofore, one of the most common methods and apparatus used for coupling a pulse transmitter in a local area network has been a pulse transformer.
There are a number of disadvantages in using a pulse transformer in a local area network. Included among the disadvantages is the fact that a typical pulse transformer is relatively expensive. Another and very important disadvantage is the fact that a pulse transformer is a low impedance type device and, consequently, a transceiver which uses a pulse transformer to couple the transceiver to a local area network has a low output impedance at all times, both when transmitting and when not transmitting.
The fact that a pulse transmitter which uses a pulse transformer has low output impedance at all times places significant constraints on a local area network in which such transmitters are used. For example, the use of pulse transmitters having a low output impedance significantly affects the signal-to-noise ratio of signals on a line, particularly when a plurality of such transmitters are coupled to the same line. As more and more of such transmitters are coupled to the line, the signal-to-noise ratio drops until a point is reached whereat communications between transceivers becomes unreliable, particularly if they are widely separated. As a consequence, such transceivers are generally required to have a relatively high output power and sensitivity which in turn makes them expensive. Alternatively, the number of such transceivers which can be coupled in a single network must be severely restricted, which seriously restricts the usefulness of the network.
The type of pulses used in a local area network also affect the performance of the network. Heretofore, a common type of pulse transmitter which has been used in local area networks and other types of communication networks has been a unipolar pulse transmitter. A unipolar pulse transmitter is a type of transmitter which generates pulses which are either positive or negative with respect to a reference potential. For example, a positive unipolar pulse is a pulse which rises and falls between a reference potential, e.g. 0 volts, and a positive potential, e.g. +5 volts. A negative unipolar pulse is a pulse which rises and falls between a reference potential, e.g. 0 volts, and a negative potential, e.g. -5 volts.
A principal disadvantage of using unipolar pulse transmissions in a communications network is associated with a phenomena known as DC wander or DC drift. Most commonly, DC wander is manifested by pulses which drift, over time, from a predetermined reference potential.
To eliminate DC wander, it has become the practice to use in conventional digital telephone systems and local area networks other pulse generating techniques such as, for example, a ternary technique based on three coding scales. One such technique is the bipolar pulse generation technique known as Alternate Mark Inversion.
In Alternate Mark Inversion, also known as AMI, logical "1's" separated by one or more logical "0's" are represented by alternate positive and negative pulses. The intervening logical "0's" are represented by the absence of a pulse. For example, a typical AMI pulse stream representing logical 10101 comprises in sequence relative to a reference clock a positive pulse, no pulse, a negative pulse, no pulse and a positive pulse.
One of the principal features of a local area network is the ability of two or more transceivers to commence transmissions simultaneously. This is possible because the only connection between the transceivers is the data transmission line itself. There are no other interconnecting control lines. When two or more transceivers commence transmissions simultaneously, the data signals on the transmission line may conflict, i.e. collide, in such a manner that the signals intended for a particular receiver become garbled or are not detectable by the receiver. To prevent such transmissions from being irretrievably lost, various proposals have been made to detect the collision of data and retransmit the colliding data. Such proposals are commonly called data collision detection and correction methods and apparatus.
One proposal which has been made for data collision detection involves comparing a transmitted signal with a received signal. Since both the transmitter and receiver of each transceiver are coupled to the transmission line, any signal transmitted by a transceiver is received by the same transceiver. After a predetermined delay due to inherent circuit delays, the signals are compared. If there is no other signal on the line, there will be a predetermined correspondence between the signals. However, if another signal is on the line, the other signal will distort the transmitted signal and this correspondence will not be present. In the absence of the desired correspondence, the transmitter will terminate its transmissions and recommence its transmissions at a later time.
The lack of correspondence between the compared signals described above can also occur if there is a mismatched impedance on the line due to a short circuit, low impedance or other circuit condition which gives rise to a significant reflected signal. Since a reflected signal will obviously have a transit time in most cases which does not correspond to the inherent circuit delay described above, a reflected signal will appear to a transmitting transceiver as if another transceiver is also transmitting. This will cause the transmitting transceiver to terminate transmissions. For this reason, it has been the practice, heretofore, to match every component coupled to the transmission line to the characteristic impedance of the line. Needless to say, such matching is particularly difficult especially in a relatively high density network when pulse transformers are used to couple the transceivers to the transmission line.